Propellent grains



Nov. 5, 1963 K. E. RUMBEL ETAL 3,109,375

PROPELLENT GRAINS Filed Dec: 7, 1956 6 Sheets-Sheet 1 INVENTORS AE/fi/ t. 04455; "Q/

Nov. 5, 1963 K. E. RUMBEL ETAL 3,109,375

PROPELLENT GRAINS 6 Sheets-Sheet 2 Filed Dec. 7, 1956 II. A

w m: E L C wr w 1 B y 5 J 6 5 Wm w Z a M AGENT Nov. 5, 1963 Filed Dec. 7, 1956 9 1N VENTORS @4462? AGENT Nov. 5, 1963 K. E. RUMBEL ETAL 3,109,375

PROPELLENT GRAINS Filed Dec. '7, 1956 6 Sheets-Sheet 5 INVENTORS Nov. 5, 1963 K. E. RUMBEL ETAL 3,109,375

- PROPELLENT GRAINS Filed Dec. '7, 1956 ll 5 W INVENT Af /m @4455: W Arc/l 650019106X AGENT United States Patent ginia Filed Dec. 7, 1956, Ser. No. 627,072 20 Claims. (Cl. 102-98) This invention relates to new and improved solid propellent grains characterized by increased effective burning rates, the extent of which can be controlled within limits and more particularly, to propellent grains having effective burning rates which are many times higher than the normal burning rates of the propellent compositions.

There is an ever-growing requirement, as for example, in the field of rocketry, for the development of propellent grains which provide incr iased propulsive performance. One way of accomplishing this is to increase the loading density; that its, to fill a greater fraction of the rocket motor chamber volume with the propellent grain. In so doing, however, an adequate rate of generation of propulsive gases must be maintained. Although solid endburning grains are notable for their high loading density, their use in propulsive devices, as for example, solidpropellent rockets, has been limited by a low rate of generation of propulsive gases. The rate of generation of propulsive gases is proportional to the product of the propellent burning rate and the burning surface area. Although there are various expedients which can be employed to increase the burning rate of the propellent material, the propellent burning rates obtained by modification of the propellent compositions hitherto have not been sufiicient to permit the general use of solid end-burning propellent grains.

Instead, it has generally been necessary to employ propellent grains having a burning surface area much greater than the grain cross section by resorting to such devices as extensive perforation of the propellent grain, concentric, spaced tubular arrangement of the propellent material, cruciform shapes and the like. Though providing the desired large area of burning surface, these expedients possess the disadvantage of weakening the grain so that the solidpropellent material must meet stringent requirements as to strength and other physical properties, which impose rigid limitations as to the type of material which can be used. In many cases, also, such grains must be provided with special external supporting and bracing structures.

Solid, end-burning grains, on the other hand, possess the strength inherent in a structure which is solid throughout and can be supported externally by the walls of the chamber of use. As compared to perforated grains, operating temperature limits of solid end-burning grains are broader, and propellent materials giving higher impulse can be employed without danger of weakening the physical structure of the grain.

Thus an increase in the effective or mass burning rate of solid end-burning grains which is sufiiciently high to bring the rate of gas evolution within the desired range makes possible the use of such grains, with their attendant advantages, for many applications where they could hitherto not have been considered. Furthermore, the use of such rapid-burning propellents, combined with other expedients for increasing burning surface, such as perfora- PatentedNov. 5, I963 tions, provides a considerably higher rate of gas evolution that could hitherto be achieved.

The object of this invention is to provide propellent grains having greatly increased effective burning rates.

Another object is to provide propellent grains, the effective burning rates of which can be controlled within limits.

Other objects and advantages will become obvious from the following detailed description.

In the drawings:

FIGURE 1 comprises a series of duplications of high speed motion picture frames.

FIGURE 2 comprises duplications of high speed motion picture frames of three propellent strands A, B, and C.

FIGURE 3 is a sectional perspective of a solid, endburning grain showing a random dispersion of short lengths of coated wire.

FIGURE 3a is an enlarged fragmentary section of the grain shown in FIGURE 3 showing the coated wires.

FIGURE 4 is a sectional perspective of a solid, endburning grain showing a dispersion of short lengths of coated wire which are longitudinally oriented.

FIGURE 5 is a sectional perspective showing a solid end-burning grain with a single continuous coated wire.

FIGURE 6 is a transverse cross-sectional view taken along line 6-6 of FIGURE 5.

FIGURE 7 is a sectional perspective showing a solid, end-burning grain with a plurality of continuous coated w1res.

FIGURE 8 is a plan view of the grain of FIGURE 7.

FIGURES 9 and 10 are sectional perspective views of other embodiments of our invention.

FIGURE 11 is a plan view of a solid, end-burning grain with preshaped ignition surface.

FIGURE 12 is a cross-sectional view taken along lines 12-12 of FIGURE 11.

FIGURE 13 is a sectional perspective of another embodiment.

FIGURE 14 is a plan view of the grain of FIGURE 13.

FIGURE 15 is a plan View of a solid end-burning grain containing a coated continuous, axially-embedded wire and coated continuous, concentric tubular metal heat conductors and having a preshaped ignition surface.

FIGURE 16 is a cross-section taken along line 1616 of FIGURE 15.

FIGURE 17 is a seectional perspective of a perforated grain with radially disposed coated continuous wires.

FIGURE 18 is a transverse cross-section taken along lines 18-18 of FIGURE 17.

FIGURE 19 is a sectional perspective of a perforated grain with longitudinally disposed coated continuous wires.

FIGURE 20 is a cross-section along lines 29-20 of FIGURE 19.

FIGURES 21 and 22 are sectional perspectives of still other embodiments of our invention.

In co-pending Keith E. Rumbel et a1. application, Serial Number 514,254, filed June 9, 1955, it is disclosed that effective or mass burning rate can be greatly increased by embedding within the propellent grain a metal heat con- 1 ductor in the form, for example, of fine wire, filaments, strips and the like. The heat conductor, which can be any metal having a substantially higher thermal diffusivity or conductivity than the propellent material, is dispersed in the propellent matrix in the form of discontinuous short wires or filaments or, preferably, in the form of a conu tinuous wire or strip oriented longitudinally in the desired direction of flame propagation. The increased burning rate of the propellent grain is due to the fact that the metal heat conductor, having a considerably higher thermal diffusivity or conductivity than the propellent material or its gaseous combustion products, effects rapid heat transfer from the high temperature combustion gases in the flame zone to unburned propellant within the grain so that the flame propagates rapidly along the metallic heat conductor. As a result, the burning surface propagates along the heat conductor at a much faster rate than the normal propellent burning rate; the burning surface recesses to form a cone with the heat conductor at its apex, thereby becoming much larger than normal; and the efiFective burning rate of the propellent grain is greatly increased.

The increase in effective or mass burning rate of a given propellant is largely determined by the particular metal used as the heat conductor. The properties of the metal which are apparently involved in determining its efficacy are thermal difiusivity, thermal conductivity and melting point. The higher the thermal diffusivity and conductivity of the metal, the more rapidly it conducts heat to the unburned portion of propellant and the more rapid is the burning rate along the wire. In a particular set of controlled tests, Ag, which has a high thermal diffusivity of 1.23 cmF/sec. at 650 C., effected an increase in burning rate of 430% whereas Pt, with the considerably lower thermal diffusivity of 0.35 cm. /sec. at 650 C., increased burning rate by 190%. Higher melting points also increase efiicacy of the metal as shown in the same set of tests by a comparison of Cu and Al. Al melts at a much lower temperature than Cu and, despite a somewhat higher thermal diffusivity, increases burning rate along the wire to a considerably lesser degree. Similarly tungsten, which has about the same thermal diffusivity as magnesium but a much higher melting point, is considerably more effective in increasing burning rate. Apparently the higher the melting temperature of the wire, the longer is the length of the wire which projects into the fiame zone, thereby providing a greater area for heat transport from the hot gases to the wire.

The embedding of continuous silver and copper wires in end-burning propellent grains has made possible increases in effective burning rate of as much as 400 to 450%. This is an enormous improvement as compared with burning rates previously obtained. However, fairly definite limits in burning rate increases obtainable by incorporation of bare metal heat conductors are set by the physical characteristics of the metal, namely, heat diffusivity, heat conductivity and melting point. It is doubtful, therefore, whether much higher rates can be achieved by this method.

There are situations, as, for example, in the field of rocketry, requiring for optimum performance both the high loading density obtainable with end-burning propellent grains and even higher effective burning rates than those made possible by the incorporation of bare metal heat conductors. There are also situations where it is desirable to employ end-burning grains having burning rates which are substantially higher than that of the propellent matrix but not as high as those ordinarily obtained by incorporating the bare metal heat conductors. Such cases can, to a considerable extent be handled by proper choice of metal or metal alloy and adjustment in thickness of conductor but tailoring of the burning rate in this way occasionally poses some practical difficulties.

We have discovered that by coating the bare metal heat conductor with a self-oxidant composition having a higher burning rate than the propellent matrix before embedding it in the propellent grain increases the effective or mass burning rate of the propellent grain. The extent of such increase in mass burning rate with a given metal conductor appears largely to be determined by the relative burning rates of the matrix and coating, the higher the burning rate of the coating relative to the matrix the greater generally being the increase in burning rate of the propellent grain. End-burning grains having effective burning rates several times greater than those obtained with the bare metal conductor can be made in this way.

We have also discovered that coating the metal heat conductor with a self-oxidant composition having a lower burning rate than that of the propellent matrix in which the coated metal is embedded, reduces the eifeotive burning rate of the propellent grain relative to that obtained with the given bare metal. Choice or proper formulation of a coating for the metal heat conductor having a sufiiciently lower burning rate than the propellent matrix thus makes possible the manufacture of propellent grains having controlled effective burning rates intermediate the normal burning rate of the propellent matrix and the burning rate of the same matrix containing the same bare or uncoated metal heat conductor similarly embedded therein.

Broadly speaking, therefore, our invention comprises embedding within the matrix of a propellent grain a metal heat conductor, such as metal wire, coated with a selfoxidant composition having a different normal burning rate from that of the matrix material, and thereby increasing the eflfective burning rate of the propellent grain to a substantially controlled degree, the increased burning rate either being intermediate the normal burning rate of the propellent matrix and the effective burning rate of the matrix containing the same bare metal heat conductor or higher than the latter.

The embedded coated metal heat conductors possess still another important advantage since they afiord a means not only of considerably increasing the effective burning rate of the propellent grain but also of simultaneously decreasing the burning rate pressure exponent of the grain. In many cases such reduction in pressure exponent is desirable since it decreases sensitivity of the burning rate to changes in pressure.

In the case of embedded bare metal heat conductors, such as wires, at pressures of 600 psi. and higher, the pressure exponent decreases with increase of wire thickness and above a certain thickness, which varies with the particular metal and propellent material, the pressure exponent becomes even less than that of the propellent matrix itself. We have found that in many instances, the self-oxidant coating effects even greater reduction in pressure exponent than that accomplished by the same bare conductor. This reduction is frequently very considerable.

By self-oxidant coating composition as employed in this specification and claims is meant a composition which contains within itself oxygen available for combustion of a fuel component of the composition. This ox gen can be present in the form of an oxidizing agent which provides oxygen for the combustion of a separate fuel component, such as an organic fuel or a finely divided metal, or it can be present in the form of compounds which contain sufiicient oxygen available for substantial internal combustion of oxidizable portions of the molecule.

For effective, eflicient and reliable action, it is desirable that the coating composition adhere to the metal heat conductor in the form of a continuous, homogeneous film, which, preferably is sufficiently elastic or non-brittle not to chip or crack. This can generally be accomplished by employing active components which form a sufi'iciently plastic composition for coating purposes or by incorporating a binder which serves both to hold the active components together in a homogeneous mixture and to impart suitable coating properties.

Coating compositions having different burning rates suitable for different propellent applications can readily be formulated by proper selection of fuel and oxidizer components, by varying the concentrations of the fuel and oxidizer, by the incorporation of catalysts which influence the burning rate, by increasing or decreasing the particle size of certain types of fuels, such as powdered metals, or of certain types of oxidizing agents, such as solid inorganic oxidizers, and the like. The suitability of a given coating composition for use with a particular metal conductor and a particular propellent matrix to obtain a desired burning rate can readily be determined by the preparation and burning of test grains or strands by methods well known to those skilled in the art.

The coating compositions can comprise, in Whole or in part, compounds which are internally oxidant, namely, contain suflicient molecularly combined oxygen available for substantial internal oxidation of carbon and hydrogen components of the molecule, such as nitrocellulose, nitroglycerin, nitrot-oluene and the like. Good coating compositions can be prepared by gelatinizing nitrocellulose with an internally oxidant plasticizer, such as nitroglycerine, or with any other suitable organic plasticizer which is inert, namely not internally oxidant. Examples of suitable organic plasticizers include triacetin, the various phthalates such as diethyl phthalate, dibutyl phthalate, dioctyl phthalate, di-(methoxyethyl) phthalate, methyl phthalyl ethyl glycolate, ethyl phthalyl ethyl glycolate and butyl phthalyl butyl glycolate, sebacates such as dibutyl and dioctyl sebacates, adipates such as dioctyl adipate and di-(3,5,5-trimethylhexyl) adipate, glycol esters of higher fatty acids, organic phosphate esters such as tributoxyethyl phosphate, and the like.

Burning rate of such coating compositions can be controlled within limits by choice of the particular internally oxidant compound or compounds, varying the concentration of such compounds in an inert plasticizer, incorporation of an external oxidizing agent where the internally combined oxygen is not suificient for complete oxidation of the carbon and hydrogen components of the molecule or Where an inert plasticizer is employed in whole or in part.

Coating compositions comprising a fuel which requires an external source of oxygen for combustion and an oxidizing agent are particularly suitable for our purpose since they make possible the formulation of compositions having a wide range of burning rate. The fuel can be any readily oxidizable material including organic fuels, finely divided metals, carbon, silicon and the like employed alone or in combination.

We have found that coating compositions containing finely divided metal as the fuel component are especially good. Any metal which forms stable compositions with an oxidizing agent at ordinary temperatures and which forms oxides that are stable at elevated temperatures, can be employed for our purpose. The finely divided metal fuel is particularly effective where high propellent burning rates are desired. Finely divided metal fuels characterized by substantially higher exothermic heats of reaction than carbon when oxidized to carbon monoxide generally provide compositions having higher burning rates than the propellent matrix. Such metals include, for example, Al, Be, B, Ca, Ce, Mg, Si, Ti, V, Zn and the like. Finely divided metals having substantially low to moderate exothermic heats of oxidation, such as Cu, Fe, Ni, Mn, Or and the like can be employed as fuels where the effective propellent burning rate desired is intermediate that of the propellent matrix alone and that obtained with the same uncoated wire.

Burning rates and flame temperatures of the coating compositions are determined to a substantial degree by the particular metal used, the amount of metal incorporated and also to some extent by the particle size of the metal. Generally speaking, the smaller the metal particles the higher is the burning rate within limits. The

burning rate of the coating composition is also influenced by the presence of other fuel components such as organic materials.

The finely divided metal fuel and the requisite oxidizing agent can be formulated into a suitable, homogeneous coating composition by admixture with a suitable binding agent which imparts adequate film-forming properties and adherency when the composition is applied to the metal heat conductor. Preferably, the binder is an oxidizable organic material. In such case, the binder can also serve as a fuel component in addition to the metal although it is not essential that sufficient oxidizer be incorporated to oxidize all of the metal and organic fuel or hinder. Suitable organic binders which provide desirable coating properties and which can also serve as fuels include, for example, organic polymers such as cellulose esters, cellulose ethers, vinyl polymers, acrylates and methacrylates, phenolaldehydes, urea-formaldehydes, polyamides, and the like; natural and synthetic rubber; natural resins; organic silicones such as dimethyl siloxane; bituminous materials such as pitch and asphalt; and the like.

In the case of the synthetic polymers, it is frequently desirable to incorporate a non-volatile organic plasticizer to improve the workability and film-forming properties of the plastic and the physical properties of the coating in terms, for example, of reduced brittleness and increased adherency. The organic plasticizer can also serve as a fuel component in the coating composition. Any organic plasticizer which is compatible with the polymer and imparts the desired physical properties can be used. Plasticizers which are suitable for the various polymers include, for example, phthalates such as dimethyl phthalate, diethyl phthalate, dibutyl phthalate, dioctyl phthalate, di(methoxyethyl) phthalate, di-(ethoxyethyl) pht-halate, trnethyland ethyl-phthalyl glycolate, butyl phthalyl butyl glycolate, sebacates such as dibutyl and dioctyl sebacate, adipates such as dioctyl adipate, acetates such as glyceryl triacetate, butylene glycol diacetate and cresyl glyceryl diacetate, glycol esters of higher fatty acids, citrates such as triethyl citrate and acetyl triethyl citrate, organic phosphate esters such as tri-butoxyethyl phosphate and trimethyl phosphate, maleates such as methyl maleate, propionates such as diethylene glycol propionate, and the like.

Organic materials can be employed very effectively as the sole fuel component in combination with an oxidizing agent. Any readily oxidizable organic material can be used for the purpose including for example, synthetic and natural polymers, natural and synthetic rubber, bituminous materials such as pitch and asphalt, Waxes, carbohydrates and the like. In general, we prefer to employ organic fuels which possess, in addition, good bonding, film-forming and coating properties such as various organic polymers and rubber, both natural and synthetic. Synthetic polymers which are particularly good for our purpose include, for example, vinyl polymers such as polyvinyl chloride, fatty acid cellulose esters such as cellulose acetate, cellulose ethers such as ethyl cellulose, acrylate and methacrylate esters, polyamides and the like. The polymers are preferably employed with suitable non-volatile organic plasticizers, illustrative examples of which have been set out above.

Burning rate of coating compositions containing organic material as the sole type of fuel is determined largely by the particular material selected, the specific oxidizing agent employed and the amount and particle size of the oxidizer. Generally speaking, other factors being constant, increasing burning rates are obtained with increasing concentrations of oxidizer up to stoichiometric levels.

Burning rates as high as those obtainable with a finely divided metal fuel having a considerably higher exothermic heat of oxidation than carbon to carbon monoxide cannot generally be achieved when an organic material is employed as the sole type of fuel. However, such organic fuel coating compositions for the metal heat conductor can be prepared having substantially higher burning rates than the propellent matrix of the propellent grain with resulting substantially increased mass burning rate of the grain as compared with that obtained with the bare metal heat conductor. Another important advantage of organic fuel coatings lies in the fact that such compositions having lower burning rates than the propellent grain matrix can be easily prepared as, for example, by reducing oxidizer concentration. Application of coatings having lower burning rates than the propellent matrix to the metal heat conductor results in an over-all mass burning rate for the propellent grain which is intermediate that of the matrix alone and that of the matrix containing the embedded bare metal, thus providing effective burning rate control within this area.

Any finely divided oxidizing agent which provides readily available oxygen for combustion of the fuel compo nent of the coating composition can be employed including, for example, ammonium, K, Na and Li chlorates and perchlorates, ammonium, K, Na and Ba nitrates, metal oxides, peroxides or superoxides, such as those of Cu, Fe, Hg, Pb, Mo, Co, Ni, Zn, As, Sb, Sn, Ba, K, Mn and Na, and the like. Although we prefer the inorganic oxidizing salts and oxides because of their relative stability, organic oxidizing agents such as hexa-nitroethane, mannitol hexanitrate, cyclotrimethylene trinitramine and the like can also be used if satisfactorily desensitized.

The concentration of oxidizer incorporated in the coating composition is not critical and is determined largely by the requirements of the particular fuel components and the burning rate desired. It can be introduced in some cases in an amount as low as to and as high as 85% or more.

The thickness of the coating on the metal heat conductor influences to some extent the over-all or effective burning rate of the propellent grain but is not critical.

The coating compositions can be prepared and applied to the metal heat conductor in any desired manner as, for example, by dipping or spraying. In some cases it may be desirable to fiuidify the coating prior to application by addition of a volatile solvent which is removed by volatilization after the coating is applied. Where heating is required as, for example, for curing or solution of a polymer in plasticizer to form a solid gel, temperatures substan tially below ignition temperature should be maintained.

The metal heat conductor, which is preferably copper or silver, although it can be any other metal having good heat conductive properties, such as platinum, steel, tungsten, aluminum, magnesium and the like, after being coated with the self-oxidant composition as aforedescribed, is embedded Within the matrix of the propellent grain so that the entire surface of that portion of the coated metal which lies within the body of the propellent grain is in intimate contact with the propellent matrix. This intimate contact is essential to effectuate control of the burning rate of the matrix by the embedded coated wire.

The metal heat conductor employed can be in the form of Wire of any cross-sectional shape, or thin strips which are flat or bent into shapes such as, for example, tubes, wedges and the like. The strips can be solid or perforated as, for example, in the form of wire screening. The use of wire is our preferred embodiment for the practical reason of its more common availability. Although the following description will be given in terms of the use of wire, it will be understood that similar results are obtained with metal heat conductors of other shapes as aforedescribed such as thin strips, tubes, or the like. The term wire as employed in this specification and claims refers to elongated metal filaments which are not newssarily circular in cross section but which can also be of other crosssectional shapes such as rectangular, oval or the like.

It will also be understood that the term coated or coating as employed hereafter refers to the self-oxidant coatings aforcdescribed.

When a coated metal wire is embedded in a solid propellant, and the grain ignited, the propellant burns at its normal burning rate for a short distance beyond the beginning of the wire. The exposed metal is heated to a high temperature by the hot gases and this heat is then conducted by the wire into the unburned portion of the wire coating and the propellant. Burning then proceeds rapidly along the coated wire. The burning surface adjacent to the coated wire recesses to form a cone with the wire at its apex. An equilibrium point is very rapidly reached where the angle of the vertex of the flame zone at the Wire, and thus the burning rate along the wire becomes substantially constant. Propagation of the burning surface continues at a high rate along the wire. The rate of gas evolution is greatly increased by the large increase in burning surface produced by recessing along the wire.

FlGURE 1 illustrates graphically the burning phenomenon which occurs when a coated metal wire is embedded in solid propellant. The series shown are duplications of frames selected from a hight speed motion picture of the actual burning of a propellent strand.

T he matrix in this case comprised 74.63% ammonium perchlorate, 12.44% polyvinyl chloride, 12.44% dibutyl sebacate and 0.5% stabilizer. The 5 mil diameter silver wire was coated with a composition comprising 72.75% NH ClO 5.8% polyvinyl chloride, 1.45% Hycar 1312 (butadiene-acrylonitrile copolymer) and 20% Al powder. The coated wire was longitudinally embedded in the strand, which was 2 mm. thick, 6 mm. wide and 40 mm. long. The embedded coated wire terminated at short distance from the initial ignition surface at one end of the stran The propellant was burned in a nitrogen atmosphere at 1015 p.s.i.a.

Elapsed burning time is indicated at the bottom of each frame. In frame A at time zero, namely at initiation of ignition, the burning surface is plane. It is still plane when the frame front reaches the beginning of the wire, as shown in frame B after burning a distance of 0.143 in. in 0.309 sec. at the normal burning rate of the propellent matrix of 0.46 in./sec. In frame C, after an elapsed time of 0.347 sec. the burning surface is just starting to propagate along the wire with consequent recessing. The plane burning surface continued beyond the beginning of the coated wire for a distance of 0.0177 in. before recessing began. Thereafter, as shown in frames D and E, the burning surface propagates rapidly along the coated wire with continued recessing until the angle subtended by the burning surface and the coated wire reaches an equilibrium value which remains substantially constant. At this point the burning rate along the coated wire also becomes substantially constant. The equilibrium cone angle is formed with extreme rapidity as shown in frame D where it has already become established.

The great increase in burning rate along the coated wire is clearly shown by a comparison of the burning distance of 0.1607 in. and elapsed time of 0.347 sec. between frames A and C and the burning distance of 0.224 in. and elapsed time of 0.055 sec. between frames D and E. The large increase in burning surface produced by the cone can also be seen. The effective burning rate of the propellent grain in this case was increased from 0.46 in./sec. to 3.44 in./sec. by the embedded coated wire.

The over-all burning pattern produced by a coated wire, as illustrated in FIGURE 1, is substantially similar to that produced by a bare wire with the exception that the burning rate along the same wire, after coating, in the same propellent matrix is higher when the coating has a higher normal burning rate than that of the matrix and lower when the coating has a lower normal burning rate. This phenomenon is graphically illustrated in A, B and C of FIGURE 2 which are duplications of frames selected from high speed motion pictures of the actual burning of three propellent strands which were identical except that the axially embedded 5 mil silver wire was uncoated in A and coated in B and C with self-oxidant compositions having higher burning rates than that of the propellent matrix which had the same composition as the matrix employed in the strand of FIGURE 1. The coating in B, comprising 72.75% NH ClO 5.8% polyvinyl chloride, 1.45% Hycar 1312 (liquid synthetic rubber comprising a butadiene-acrylonitrile copolymer), and 20% aluminum powder, has a lower burning rate than the coating in C, comprising 52.4% NI-I CIO 6.08% polyvinyl chloride, 1.52% Hycar 1312 and 40% zirconium. The end of the wire in each case was spaced some distance from the initial ignition surface of the propellent strand. The duplicated frames were taken at the same elapsed time after propagation of the flame along the Wire had begun and the angles subtended by the burning surfaces and the coated and uncoated wires had become constant.

Burning progressed considerably further along the coated wire in B than along the bare wire in A and even more rapidly in C than in B, because of the higher burning rate of the applied coating in C. The measured effective burning rate for strand A was 2.26 in./sec., for strand B, 3.44 in./sec. and for strand C, 5.44 in./sec. at 1015 p.s.1.a.

As shown in FIGURE 2, increasing rates of burning along the metal conductor result in decreasing cone angles at the apex. The smaller the cone angle, the deeper is the cone and the larger is the available burning surface area with concomitant increase in effective or mass burning rate of the propellent grain. The rate of burning along the metal conductor appears largely to be determined by the characteristics of the particular metal, namely its heat diffusivity, heat conductivity and melting point, and by the burning rate of the particular coating composition.

As aforementioned, before active propagation of the flame along the Wire occurs, the matrix burns along the wire at substantially its normal burning rate for a short distance until the metal is heated to a sufiiciently high temperature to ignite the unburned coating and propellant along its path. For effective action, therefore, the coated Wire must be of suflicient length both to provide for the initial exposure in the flame zone and for propagation of the flame for some distance into the unburned propellant in which it is imbedded. In general, the minimum wire length required to achieve appreciable increase in effective burning rate is about 0.05 to 0.1 inch and, preferably, about 0.2 inch.

Substantial increases in burning rate are obtained by dispersing short lengths of coated wire in the propellent matrix. Dispersion can be accomplished, for example, by mixing the short lengths of the coated wire with the propellent material prior to extrusion or casting. The wires in prope llent grains prepared in this manner generally assume a more or less random pattern as shown in FIGURES 3 and 3a where metal wires 1 having coating 4 are embedded in propellent grain 2. It will be noted, as shown in the drawing, that a large number of the randomly dispersed wires are at an angle substantially less than 180 relative to the plane of the initial ignition surface. The burning surface regenerates along such angled wires to produce recessing and increased burning surface area. Somewhat improved results in terms of increased burning rate can be achieved by orienting the dispersed short wires in the direction of flame propagation, namely substantially normal to the initial burning surface. Such a grain is shown in FIGURE 4 where coated wires 1 are embedded in propellent grain 2 having initial burning surface 3'.

The coated wires dispersed in the propellent matrix should be at least about 0.05 in. long to provide sufficient length for initial exposure into the flame zone and flame propagation along the wire, as aforedescribed. Additional improvement can be obtained by increasing the length of the dispersed wires as, for example, to about 0.2 inch or ionger. To some extent wire lengths will be determined by the size of the propellent grain. In the 10 case of large grains, for example, coated wires 2 inches long or longer can be incorporated.

The amount of discontinuous coated wire introduced into the propellent matrix is not critical, although this is one of the factors which determines the specific increase in burning rate obtained. In other words, even the addition of a very small amount will effect some increase. In most cases, it is desirable to add at least about 0.5% and, preferably, at least about 1% by weight of the propellant to obtain appreciable results. In general, the larger the quantity of coated wire of a given length added, the higher will be the effective burning rate. However, since the addition of the short Wires involves the introduction of substantially inert material into the propellant, thereby decreasing the gas-generating potential, in practice the amount incorporated will be controlled to a considerable extent by this factor. For this reason, it will generally be undesirable to add more than about 5 to 10% by weight of the propellant although, in some cases, larger amounts may be feasible.

Although substantial increases in effective burning rate can be achieved by the dispersion of discontinuous, short coated wires in the propellent matrix, vastly improved performance is obtained with the use of continuous coated wire which is longitudinally disposed in the desired direction of flame propagation. Increases in burning rate of the propellent grain which are several-fold greater than that obtained with dispersed, discontinuous, short lengths of wire can be achieved in this way despite the use of considerably smaller proportions of metal. Apparently the reason for the large disparity in performance stems from the fact that, in the case of the discontinuous wires, the flame propagates rapidly along each short length but is slowed substantially to the normal burning rate of the propellent material when it must bridge the gap between the end of one wire and an adjacent wire. With a continuous wire the flame continues to propagate rapidly and uninterruptedly through the entire length of the desired burning distance. Another important advantage of the continuous wire is that it requires the introduction of a minimum amount of inert material, generally no more than a fraction of one percent by weight of the propellant. v

FIGURE 5 shows an end-burning grain 10 containing continuous wire 11, having coating 8, axially embedded in the grain. The wire, which is normal to the initial burning surface 12, is disposed longitudinally in the direction of flame propagation as shown by the arrow and is continuous throughout the distance of flame propagation, in this case the full length of the grain. The surfaces of the grain other than the end burning surface 12 can be inhibited in any desired fashion. FIGURE 6 is a cross-sectional view of the propellent grain shown in FIGURE 5. The mode of burning of such a grain is substantially as shown in FIGURE 1. If desired, end 16 of the grain can be left uninhibited and burning instituted from both ends. The flame then propagates along the wire from both ends with doubled rate of gas evolution.

As shown in FIGURE 1, the burning surface of the grain shown in FIGURE 5 recesses as the flame propagates along the coated wire to form a cone with the wire at its apex. As the flame proceeds along the wire, the flaring end of the lengthening cone increases in width and encompasses more and more of the crosssectional area of the grain. If the grain is sufficiently narrow, the cone will eventually encompass the entire width of the grain and rapid burning of all the propellent material will continue until the other end of the wire is reached at which point only a small peripheral portion of the propellent material adjacent the end of the wire remains unburned.

In many cases, particularly where the propellent grain has a relatively large cross-section area, it is desirable to l 1 embed a plurality of continuous coated Wires at spaced intervals as shown in FIGURES 7 and 8. For example, if a grain which is short relative to its width contains only a single coated wire such as shown in FIGURE 5, the peripheral portion of unburned propellant remaining when burning has progressed the full length of the Wire may be considerably larger than desirable. This can be avoided by introducing a plurality of coated wires as shown in FIGURES 7 and 8.

It is frequently desirable to achieve equilibrium pressure, namely the point at which burning surface area and, consequently, rate of gas evolution, becomes substantially constant, as quickly as possible. Establishment of equilibrium can be hastened in several ways.

The use of a plurality of coated wires as shown in FIGURES 7 and 8 increases greatly the rapidity with which the equilibrium burning surface area can be accomplished. In the case of a single wire, the burning surface presented by the cone continues to increase in area until the flaring end intersects the peripheral edge of the grain or until burning reaches the end of the wire, as, for example, in the case of a grain which is short relative to its Width. Rate of gas evolution continues to increase until surface area of the cone becomes constant. Such high progressivity can be advantageous for some applications but not where rapid establishment of a constant burning surface area is desirable. Where a plurality of continuous coated wires are used, the cones incident to each wire soon intersect at their flaring ends and from this point on, the burning surface area remains constant as the flame proceeds along the wires.

The equilibrium state can also be established more rapidly by exposure of the coated wires a short distance beyond the initial ignition surface. In FKGURE 5, the coated Wire terminates at the initial burning surface 12. Upon ignition, the grain will burn along the wire for a short distance at the normal note of the propellent material itself. When the exposed end of the metal becomes sulficien-tly hot to initiate propagation of the flame along the wire, the effective or mass burning rate will increase rapidly until an equilibrium maximum is reached. To initiate flame propagation along the wire more quickly, the coated wires can be embedded in the grain in such a way that the ends of the wire protrude from the ignition surface as shown in FIGURE 7 where wire ends 13 extend for a short distance beyond ignition surface 12.

Recessing the ignition surface adjacent to the coated wires, preferably in the form of cones, with the Wire exposed at the apex, as shown in FIGURES 9 and 10, has-tens establishment of the equilibrium burning surface area. Any degree of preconing which bring the initial burning surface into a closer approximation of the equilibrium burning surface than an initial plane surface results in more rapid establishment of equilibrium. Thus, equilibrium is more quickly reached by the grains shown in FIGURES 9 and 10 than by the plane surfaced grains shown in FIGURES and 7.

Most rapid establishment of equilibrium burning surface area is obtained by preconing the initial ignition surface so that it has a shaped area which closely approximates or is substantially the same as the equilibrium burning surface area with the result that equilibrium is established almost immediately after ignition. In such a grain design, the angle of the vertex of the recessed cones should closely approximate the equilibrium angle and the cones should intersect with each other and the periphery of the grain at substantially the same points at which they will intersect during burning in the equilibrium state. FI URES 11 and 12 illustrate an end burning propel lent grain having the ignition surface 12 preconed in such a way that it has a shape and surface area which is substantially the same as the equilibrium burning surface as burning proceeds along the seven spaced coated wires 11. The cones 9, which flare out from the wire exposed at the apex of each, int rsect each other and the periphery of the grain 15 to form inwardly curved ridges 24 and apical points 25.

The preshaping of the ignition surface to simulate the equilibrium burning surface of an end-burning grain is determined by such factors as the number and spacing of the continuous wires, the metal of which the wires are made, the thickness of the wire, the particular coating on the wire and the particular propellent material. angle varies, for example, with the thermal diffusivity of the particular metal, the burning rate of the coating and the burning rate of the propellent matrix. The cone angle for a given combination of factors can readily be determined by those skilled in the art and the particular grain ignition surface designed accordingly. i

The thickness of the wire or other metal heat conductor is not critical inasmuch as the increase in effective burning rate is due to the higher thermal diffusivity and conductivity of the metal relative to the propellent material. The thickness of the metal conductor does, however, influence to some degree the extent of burning rate increase. One of the practical considerations which may determine, to some extent, the thickness of the wire or other heat conductor, is the undesirability of introducing such large amounts of inert material as substantially to decrease the gas-generating potential of the propellant. From this point of view, a maximum heat conductor thickness of about 30 to 50 mils will probably be desirable in most cases.

Table I summarizes test data illustrating the large increase in mass burning rate and reduction in pressure exponent of a propellent grain achieved by longitudinally embedding a Wire coated with a composition having a higher burning rate than that of the matrix as compared with the ballistic properties of the same propellant without an embedded wire and vn'th the same wire uncoated.

*NH4ClOi-82.O%, polyvinyl chloridc8.0%, dibutyl sebacate9.43%, staPIilIigrC087%. 7

4 l 475.0 polyvinyl chl0ridel0.07 dibut lsebacate 10.0 powdered Mg/Al au5 5%. y

Table II summarizes comparative burning test results obtained with variously coated wires, uncoated wires and the matrix propellant alone. The propellent grains tested were end-burning strands containing continuous longitudinally embedded wires. Measurements given were taken at a pressure of 1000 p.s.i.a. at a temperature of F. All coatings had higher normal burning rates than that of the matrix propellant. The results show the large increase in effective burning rate achieved by use of the coated wires and the marked improvement in pressure exponent.

The embedded coated metal heat conductors are effective regardless of the specific nature or composition of the propellent matrix although the specific increase in effective burning rate will vary to some extent according to the specific propellent composition, as clearly illustrated in the test data summarized in Table II. They can be employed both with composite type propellants which comprise a fuel and external oxidizer, such as the polyvinyl chloride propellants described above, Thiokol, polystyrene and polyester type propellants and the like, with single and double base nitrocellulose propellants, etc.

The cone TABLE II r-percent Wire Coating r-percent increase Metro: 11 1 5 mil coatthick- 1, increase over a 3 wire ing ness, in./sec. over matrix inch matrix bare wire AA" 0.38 Cu A 7. 5 1, 460 168 0.32 B 0. 95 Cu O 2. 05 7, 500 0. 5 AA 0. 42 Cu D 5. 4 960 86 0. 15 AA 0. 45 011 E 4. 685 48 0. 24 AA 0. 45 011 F 6. 0 1, 076 122 0. 42 AA 0. 45 Cu G 3.0 490 0.36 AA 0. 45 Cu H 4. 1 705 O. 34 AA 0. 46 A2 I 3. 4 519 0.40 AA 0.34 Cu .1 4.0 515 0.31 AA 0. 34 Ag K 4. 25 554 0. 33 AA o. 34 Ag L 5.0 670 0.40 AA 0. 45 01.1 M 3. l 508 0.45 AA 0. 45 Cu N 3. 5 587 0. AA 0. 42 Cu O 4. 3 760 0. 36 AA 0. 38 011 P 6. 7 1, 295 0. 18 AA 0. 67 Ag Q 4. 2 678 O. 29

1 Pressure exponent of matrix at 1000 p.s.i.a. and 70 F. 2 Burnlna rate at 1000 p.s.i.a. and 70 F.

9 Pressure exponent of propellent grain with coated wire at 1000 p.s.i.a. and 70 F.

" See Table I supra. Difierences in burning rate and pressure exponent of the base matrix shown in this table are caused by such factors as difierent particle size of the oxidizer.

A. 70% N 11 0101; 25% Mg/Al powder; 5% liquid synthetic rubber (polyisobutylene- Vistanex) B. 75% N H4N0 12% polyvinyl chloride; 13% dibutyl scbacate. C. 85% NH4O104; 7.5% polyvinyl chloride; 7.5%

dibutyl sebacate; plus 1.5% copper chromite (burning rate catalyst) and 0.5% organic bentonite derivative (Bentone 34) added.

D. 68227 NH4ClO4; 3.347 polyvin lchloride- 3.34 7 160); 24.97% li/lg/Al powder. 0 y y O E. 63.9% K0104; 8.88% polyvinyl chloride; 2.22% acryiiongrile copolymcr, Hycar 1312); 25.0% 65/35 Mg/Al powder powder. I

G. Al powder; 72.75%

H. 22.33% Al powder; 70.63% NHiClol; 5.63% polyvinyl chloride; 1.41% Hycar 1312.

I. 20% Al powder; 72.75% NH ClO4; 5.80% .1. 65/35 Mg/Al powder; 63.5% K0104; 9.12% K. 25% 65/35 Mg/Al powder; L. 25% 65/35 Mg/Al powder; 69.8% M. 20% Al; 72.75% NH1OlO4; 5.80% PVC; 1.45% Hycar 1312. N. 20% Al; 72.75% NH ClO4; 5.80% PVC; 1.45% Hycar 1312.

polyvinyl chloride; 1.45% Hycar 1312.

polyvinyl chloride; 2.28% Hycar 1312. 63.5% K0104; 9.12% PVC; 2.28% Hycar 1312.

NH4OlO1; 4.16% PVC; 1.04% Hycar 1312.

O. 85% NH4ClO1;7.5% polyvinyl chloride; 7.5% dihutyl sebacate; plus 1.5% Cu chromite and 0.5% Bentone added.

P 74% NH1ClO4; 26% Mg/Al powder.

Q: 82% NH C10 8% polyvinyl chloride; 9.43% dibutyl sebacate; 1.00% copper chromite. We have found that the use of finely divided Zr as a 40 metal fuel in the self-oxidant coating is particularly effective in increasing the mass burning rate of the propellant grain and reducing the pressure exponent as shown in Table III. The data summarized in this table indicate that, in general, higher burning rates are obtained with silver wire as compared with copper. The data also show that finer particle size of the coating components, such as the metal and oxidizer, tends to increase mass burning rate, as indicated by the fact that ball milling the coating solids for 41 hours with 70 stones resulted in higher buming rates than those achieved with coatings which had been ball milled 17 hours with 35 stones.

TABLE III Ballistic Properties of Matrix AA" Containing Embedded Wire Coated With Compositions Containing Zirconium Powder Ballistic properties Percent 5 mil Zr, 1 per- Coating r of base increase wire cent thickness matrlx, of r over (inch) r, 2 a a in./sec. base 1n./scc matrix H 25 0.004 3. 25 0.23 0.52 525 a 30 0. 002 4.35 0. 23 0.58 650 e 30 0.002 4. 10 0.28 0. 58 608 a 30 0.005 5.10 0.18 0.58 780 30 0.005 4.50 0.15 0. 58 676 a 0. 003 4.10 0.58 607 B 35 0. 002 4. 90 0.16 0.58 745 B 35 0. 002 5. 20 O. 22 0. 58 797 e 35 0.005 4.50 0.30 0. 58 676 a 35 0.005 4. 75 0. 25 0.58 719 40 0.002 5. 10 0.19 0. 58 780 B 40 0.002 5. 30 0.22 0.58 815 40 0.005 5.10 0.30 0.58 780 B 40 0.005 5. 30 0.22 0.58 815 b 0.002 4. 85 0. 22 0.52 834 b 45 0. 002 4. 30 0. 27 0.52 728 b 45 0. 0045 5. 30 0.22 0. 52 920 b 45 0. 0045 4. 80 0.31 0.52 824 b 35 0.002 6.00 0.245 0.58 932 TABLE IIIC0ntinued Ballistic properties Percent 5 mil Zr, per- Coating r of base increase wire cent thickness matrix of r over (inch) 1', n 3 in./scc. base inJsec. matrix b 35 0. 005 5.10 0. 20 0. 58 780 b 40 0. 002 6. 20 0. 31 0. 58 970 b 40 0.0025 7.00 0. 25 0.58 1, 108 40 0.0045 7.60 0.096 0.58 1,210 b 40 0.005 8. 10 0. 16 0. 58 1, 296 b 40 0. 002 6.00 0. 24 0.52 1, 050 b 40 0.0025 7. 50 0.15 0.52 1, 341 b 40 o. 0045 6. 0.21 0. 52 1, 208 b 40 0.005 7. 75 0. 19 0.52 1, 390

See note Table I. Grain cured for 10 minutes at 350 F. moon in range of 0.39 to 0.46.

1 For coating formulations see Table III-A below.

2 Burning rate at 1000 p.s.i.a. and 70 F.

3 Pressure exponent at 1000 p.s.i.a. and 70 F.

a Solid coating components ballmilled 17 hours in 30% mesityl oxide with 35 stones.

b Solid coating components ballmilled 41 hours in 30% mesityl oxide with 70 stones.

TABLE IILA Zirconium... 25. 0 35. 0 so. 0 40. 0 45. 0 61.9 57. 4 62.4 52. 4 47. 4 10.48 6.08 e. 08 6.08 6.08 Hycar 1312 2.62 1.52 1.52 1.52 1.53

Matrix AA (see Table I supra) with 1% copper chromite (burning rate catalyst) added.

74.63% NHiOl 04; 12.44% polyvinyl chloride; 12.44% dibutyl sebacate; 0.5% stabilizer.

As aforementioned, the metal heat conductor, though conveniently used in the form of wire, can also be employed in the form of continuous thin coated strips which can be fiat or bent into other desired shapes such as a V-shape or a tube. The effect on mass burning rate is substantially similar to that obtained with coated wires. The burning surface along metal heat conductors which are substantially wider than they are thick, assumes the configuration of a V-shaped trough rather than the cone incident to a wire. As in the case of wires, a plurality of coated strips or tubes can be employed.

The various expedien'ts for hastening the establishment of the equilibrium burning surface, discussed above in connection with the use of coated wires, can be employed with thin, wide, coated conductors, such as protrusion from the ignition surface, and pre-troughing the ignition surface adjacent the heat conductor.

FIGURES 13 and 14 show a solid, end-b-urning propellent grain containing a coated V-shaped metal heat conductor 18 which is disposed longitudinally the full length of the grain with one end exposed at the ignition surface 12.

FIGURES 15 and 16 show a concentric tubular arrangement of coated metal heat conductors 19 with a coated wire 11 embedded axially. The ignition surface 12 is preshaped to a configuration which closely approximates the equilibrium burning surface. The flaring ends of coned recess 17 with the central Wire at its apex intersects with the circular V-shaped trough 20, which has the first concentric tube at its apex, to form a ridge 21. The outer flaring edge of this trough in turn intersects with the second trough having the outer concentric metal tube at its apex to form a second ridge. The second trough flares out to intersect with the periphery of the grain at 22.

The large increase in effective burning rate made possible by the incorporation of coated metallic heat conductors into the propellent matrix, particularly in the form of continuous wires or strips which extend substantially the entire distance of flame propagation, makes practical the use of solid, end-burning propellent grains for many applications, as for example in rocketry. This is of great importance because of their other advantages as compared with perforated grains, such as higher loading density, and greater strength. Propellants of higher impulse can be employed without danger of weakening the physical structure of the propellent grain and wider operating temperatures can be employed.

Although the preceding description has been in terms of solid end-burning grains because of the enormous improvement in burning rate and other properties, such as pressure exponent, of this type of propellent grain, our invention can also be applied very advantageously to other types of propcllent grains, such as perforated grains. The incorporation of coated metal wire into the matrix of a perforated grain results in a propellant which burns with extreme rapidity by virtue of the combination of the increased effective burning rate along the coated metal wire and the large initial burning suriace provided by the perforations. The wire can be continuous through the distance of flame propagation or can be dispersed through the matrix in the form of short, discontinuous lengths of coated wire. As in the case of end-burning grains, the continuous wires provide a considerably higher effective burning rate than the discontinuous Wire.

The continuous coated Wire can be positioned in the matrix of the perforated grain in a manner most suitable for the particular application. For example, in the grain shown in FIGURES 17 and 18, the embedded coated wires radiate out from the central perforation M- which provides the initial burning surface. With the exterior surface 15 inhibited, the flame rapidly propagates periph erally along the Wires.

FIGURES 19 and 20 show an end-burning cylindrical grain with central perforation l4 and a plurality of continuous coated wires which are normal to the end-burning surfaces 12 and 16 and run the length of the grain. If both the exterior surface 15 and the surface exposed by the central perforation are inhibited, the flame propagates rapidly along the wires from both ends of the grain. If the central perforation surface is uninhibited, the grain also burns outwardly from the central perforation but propagation of this flame front is considerably slower because of the absence of wire in the direction of flame propagation. Such grains are particularly suitable for some rocket applications since it makes possible venting of combustion gases produced at the end of the grain adjacent to the closed end of the rocket chamber through the central perforation.

As in the case of solid grains, the heat conductor incorporated into perforated grains can be in the form of coated wires or thin coated strips of metal shaped into any suitable configuraion such as Wedges, tubes, etc.

For many applications requiring the use of propellent grains, it is essential that a high burning rate be maintained throughout combustion. This requirement can be satisfied by extending the continuous coated heat conductor for substantially the entire distance of flame propagation of the grain, as shown, for example, in FIGURES 5, 7, 9, 10, 13, 17 and 19. There are some cases, however, where a very high rate of gas generation is required for only a portion of the combustion cycle as, for example, until a propelled object is air-borne, after which the rate of combustion gas production can be reduced. Such a requirement can be met by limiting the length of the coated metal conductor so that it extends in the direction of flame propagation only as far as it is desired to obtain the high rate of burning conferred by the coated conductor. After burning has proceeded along the full length of the conductor, combustion of the grain then continues at the normal rate of the propellent grain material.

FIGURE 21 shows a solid, end-burning propellent grain in which the coated heat conducting metal wires 11, which are disposed longitudinally in the direction of flame propagation with one end exposed at ignition surface 12, do not extend the full burning distance of the grain. Burning proceeds from the ignition surface at the high rate induced by the embedded coated heat conductors until the point at which they terminate within the grain, after which burning continues at the normal rate of the grain composition until the full burning distance of the grain has been traversed at end 16.

It will be understood that the various expedients aforediscussed which can be employed to regulate burning rate, pressure exponent, establishment of equilibrium pressure and the like, such as choice of particular coating compositions, metal species and thickness of the heat conductor, the use of one or a plurality of coated heat conductors, protrusion of the heat conductor from the ignition surface, preconing, perforation, etc., can be employed both where the coated heat conductor is continuous substantially throughout the entire burning distance of the grain or where it extends only for a predetermined portion of the burning distance.

In certain applications, it may be desirable to employ a propellant which progresses from a relatively low initial impulse to a high impulse. In such case, the coated metal heat conductor can be embedded in the grain at a predetermined point spaced from the initial ignition surface. The spacing can be small or considerable depending on the particular situation. An example of such a grain is illustrated in FIGURE 22 where 12 is the initial ignition surface.

Although this invention has been described with reference to illustrative embodiments thereof, it will be apparent to those skilled in the art that it can be embodied in other forms within the scope of the claims.

We claim:

1. A solid propellent grain, said grain comprising a self-oxidant, solid propellent matrix, the combustion of which generates propellent gases, and having at least one initial, exposed ignition surface, said matrix containing embedded therein an elongated metal heat conductor coated with a self-oxidant, solid coating having a normal burning rate different from the normal burning rate of the propellent matrix, said coated metal heat conductor being positioned substantially normal to the plane of said initial ignition surface of said grain and being continuously and longitudinally disposed in the direction of fiame propagation of the grain, said conductor within the body of the grain having a length of at least about 0.2 inch and a maximum metal thickness of about 0.05 inch in at least one transverse direction, the entire surface of said length of said coated metal conductor lying within the body of the propellent grain and being in intimate, gas-sealing contact with the propellent matrix, the burning surface of said grain after ignition regenerating progressively along said coated metal heat conductor and, in so doing forming a recess which is substantially V-shaped in at least one plane with said coated metal heat conductor at the apex of said recess, thereby forming a recessed surface of substantially larger surface area than that of a plane burning surface, the coated metal heat conductor thereby serving controllably to increase the mass burning rate and, thereby, the mass rate of gas generation of said propellent grain to a level determined by the particular metal heat conductor and the relative burning rates of the self-oxidant coating and the propellent matrix.

2. The propellent grain of claim 1 in which the heat conductor is continuous substantially throughout the distance of flame propagation of the grain.

3. The propellent grain of claim 1 in which the selfoxidant coating has a higher normal burning rate than the normal burning rate of the propellent matrix.

4. A solid propellent grain, said grain comprising a self-oxidant, solid propellent matrix, the combustion of which generates propellent gases and having at least one initial, exposed ignition surface, said matrix containing embedded therein a plurality of elongated metal heat conductors coated with a self-oxidant, solid coating having a normal burning rate different from the normal burning rate of the propellent matrix, said coated metal heat conductors being positioned substantially normal to said initial ignition surface of said grain, being substantially spaced from each other in the plane transverse to the direction of flame propagation, and being continuously and longitudinally disposed in the direction of flame propagation of the grain, said conductors within the body of the grain having a length of at least about 0.2 inch and having a maximum metal thickness of about 0.05 inch in at least one transverse direction, the entire surface of said length of said coated metal conductors lying within the body of the propellent grain and being in intimate, gas-sealing contact with the propellent matrix, the burning surface of said grain, after ignition, regenerating progressively along each of said coated metal heat conductors, and in so doing, forming a recess which is substantially V-shaped in at least one plane with each of said coated metal heat conductors at the apex of a recess, thereby forming a recessed surface of substantially larger surface area than that of a plane burning surface, said coated metal heat conductors being spaced sufliciently apart to permit said recessing, the coated metal heat conductors thereby serving controllably to increase the mass burning rate, and, thereby, the mass rate of gas generation of said propellent grain to a level determined by the particular metal heat conductor and the relative burning rates of the metal coating and the propellent matrix.

5. The propellent grain of claim 4 in which the selfoxidant coating has a higher normal burning rate than the normal burning rate of the propellent matrix.

6. The propellent grain of claim 4 in which the heat conductors are metal wires.

7. The propellent grain of claim 4 in which the metal heat conductors are continuous substantially throughout the distance of flame propagation of the grain.

8. The propellent grain of claim 7 in which the metal heat conductors are metal wires.

9. The propellent grain of claim 8 in which the self oxidant coating has a higher normal burning rate than the normal burning rate of the propellent matrix.

10. The propellent grain of claim 4 in which the selfoxidant coating comprises a powdered metal fuel and oxidizing agent.

11. The propellent grain of claim 10 in which the selfoxidant coating comprises a powdered metal fuel, an oxidizing agent, and an organic binder.

12. The propellent grain of claim 10 in which the powdered metal fuel is selected from the group consisting of Mg, Al, and Zr.

13. The propellent grain of claim 4 in which the heat conductor comprises metal selected from the group consisting of copper and silver.

14. The propellent grain of claim 4 in which the ends of the heat conductors are exposed at the ignition surface of the propellent grain.

15. The propellent grain of claim 14 in which the ends of the heat conductors are exposed at the apex of a recess in the ignition surface.

16. A solid propellent grain, said grain comprising a self-oxidant, solid propellent matrix, the combustion of which generates propellent gases and having at least one initial exposed ignition surface, said matrix containing embedded and randomly dispersed therein a plurality of spaced, elongated metal wires having a minimum length of about 0.05 inch and a maximum diameter of about 0.05 inch and coated with a self-oxidant, solid composition having a normal burning rate different from the normal burning rate of the propellent matrix, the entire surface of said coated metal wires lying within the body of the propellent grain and being in intimate, gas-sealing contact with the propellent matrix, a substantial number of said randomly dispersed, coated wires being at an angle, relative to the plane of said initial ignition surface, which is substantially less than the burning surface of said grain, after ignition, regenerating progressively along each of said coated metal wires positioned at said angle substantially less than 180, and, in so doing, forming a recess which is substantially V-shaped in at least one plane with a coated wire at the apex of each formed recess, thereby forming a recessed surface of substantially larger surface area than that of a plane burning surface, said coated metal wires being spaced sufliciently apart to permit said recessing of the burning surface, the coated metal wires thereby serving controllably to increase the mass burning rate and, thereby, the mass rate of gas generation of said propellent grain to a level determined by the particular metal wire and the relative burning rates of the self-oxidant cooating and the propellent matrix.

17. A solid propellent grain, said grain comprising a self-oxidant, solid propellent matrix, the combustion of which generates propellent gases and having at least one initial, exposed ignition surface, said matrix containing embedded therein elongated metal heat conductor means forming a longitudinal tubular structure within the body of said grain, said metal heat conductor means being coated With a self-oxidant, solid coating having a normal burning rate different from the normal burning rate of the propellent matrix, being positioned substantially normal to the plane of said initial ignition surface of said grain, and being continuously and longitudinally disposed in the direction of flame propagaton of the grain, said coated tubular conductor means within the body of the grain having a length of at least about 0.2 inch and having a maximum wall thickness of about 0.05 inch, the entire surface of said length of said coated conductor means lying Within the body of the propellent grain and being in intimate, gas-sealing contact with the propelient matrix, the burning surface of said grain after ignition regenerating progressively along said coated, tubular, metal heat conductor means and, in so doing, forming a recess which is substantially V-shaped in at least one plane with said coated metal conductor means at the apex of said recess, thereby forming a recessed surface of substantially larger surface area than that of a plane burning surface, the coated metal heat conductor means thereby serving controllably to increase the mass burning rate and, thereby, the mass rate of gas generation of said propellent grain to a level determined by the particular metal heat conductor and the relative burning rates of the self-oxidant coating and the propellent matrix.

18. The propcllent grain of claim 17 in which the coated metal heat conductor means forms a plurality of longitudinal tubular structures within the body of the grain.

19. The propellent grain of claim 18 in which the coated metal heat conductor means is continuous substantially throughout the entire distance of flame propagation of the propellent grain.

20, The propellent grain of claim 17 in Which the coated metal heat conductor means is continuous substantially throughout the entire distance of flame propagation of the propellent grain.

References Cited in the file of this patent UNITED STATES PATENTS 1,381,381 Buckingham Apr. 22, 1919 1,530,692 Paulus Mar. 24, 1925 2,072,671 Foulke Mar. 2, 1937 2,417,437 Nicholas Mar. 22, 1947 2,548,926 Africano Apr. 17, 1951 2,549,811 Hickman Apr. 24, 1951 2,574,479 Hickman Nov. 13, 1951 2,637,274 Taylor May 5, 1953 FOREIGN PATENTS 652,542 Great Britain Apr. 25, 1951 742,283 Great Britain ec. 21, 1955 

1. A SOLID PROPELLENT GRAIN, SAID GRAIN COMPRISING A SELF-OXIDANT, SOLID PROPELLENT MATRIX, THE COMBUSTION OF WHICH GENERATES PROPELLENT GASES, AND HAVING AT LEAST ONE INITIAL, EXPOSED IGNITION SURFACE, SAID MATRIX CONTAINING EMBEDDED THEREIN AN ELONGATED METAL HEAT CONDUCTOR COATED WITH A SELF-OXIDANT, SOLID COATING HAVING A NORMAL BURNING RATE DIFFERENT FROM THE NORMAL BURNING RATE OF THE PROPELLENT MATRIX, SAID COATED MEMBER HEAT CONDUCTOR BEING POSITIONED SUBSTANTIALLY NORMAL TO THE PLANE OF SAID INTITAL IGNITION SURFACE OF SAID GRAIN AND BEING CONTINUOUSLY AND LONGITUDINALLY DISPOSED IN THE DIRECTION OF FLAME PROPAGATION OF THE GRAIN, SAID CONDUCTOR WITHIN THE BODY OF THE GRAIN HAVING A LENGTH OF AT LEAST ABOUT 0.2 INCH AND A MAXIMUM METAL THICKNESS OF ABOUT 0.005 INCH IN AT LEAST ONE TRANSVERSE DIRECTION, THE ENTIRE SURFACE OF SAID LENGTH OF SAID COATED METAL CONDUCTOR LYING WITHIN THE BODY OF THE PROPELLENT GRAIN AND BEING IN INTIMATE, GAS-SEALING CONTACT WITH THE PROPELLENT MATRIX, THE BURNING SURFACE OF SAID GRAIN AFTER IGNITION REGENERATING PROGRESSIVELY ALONG SAID COATED METAL HEAT CONDUCTOR AND, IN SO DOING FORMING A RECESS WHICH IS SUBSTANTIALLY V-SHAPED IN AT LEAST ONE PLANE WITH SAID COATED METAL HEAT CONDUTOR AT THE APEX OF SAID RECESS, THEREBY FORMING A RECESSED SURFACE OF SUBSTANTIALLY LARGER SURFACE AREA THAN THAT OF A PLANE BURNING SURFACE, THE COATED METAL HEAT CONDUCTOR THEREBY SERVING CONTROLLABLY TO INCREASE THE MASS BURNING RATE AND, THEREBY, THE MASS RATE OF GAS GENERATION OF SAID PROPELLENT GRAIN TO A LEVEL DETERMINED BY THE PARTICULAR METAL HEAT CONDUCTOR AND THE RELATIVE BURNING RATES OF THE SELF-OXIDANT COATING AND THE PROPELLENT MATRIX. 